1. Field of the Invention
The present invention relates to a network telephone system such as an IP (Internet Protocol) telephone system for performing voice communication among telephone terminals through an IP network, and a main equipment and telephone terminals used in the network telephone system.
2. Description of the Related Art
Network telephone systems (IP telephone systems) for transmitting/receiving images or voices as packet data in real time bidirectionally through a packet transmission network have come into wide use in recent years. SIP (Session Initiation Protocol) has been used widely as a protocol of these IP telephone systems.
In this type of system, addresses of terminals (hereinafter referred to as SIP terminals) connected to a packet transmission network are registered in advance in a main equipment on the packet transmission network. When the SIP terminals communicate with each other, the main equipment authenticates the SIP terminals based on the registered addresses.
However, even if failure occurs in the packet transmission network between the main equipment and the SIP terminals, it is impossible to check states of connection between the main equipment and the SIP terminals except the case where call is made from the main equipment side or from the SIP terminal side and the case where the main equipment requests the SIP terminals to make re-registration periodically before expiration of registration. Generally, the main equipment can judge the connection states as long as the terminals connected to the main equipment are digital telephones, standard telephones or IP terminals with Keep-Alive functions. Thus, service can be established when there is an incoming call to a given terminal.
In the case of SIP terminals, it is however impossible for the main equipment to judge presence/absence states of the SIP terminals. Therefore, a method for installing a “Session Timer” relating to Keep Alive based on an SIP terminal standard into SIP terminals has been proposed recently (e.g., see U.S. Pat. No. 6,625,141). It is noted that the “Session Timer” is one of extensions of SIP, but does not refer to actual timers.